1. Field of the Invention
The present invention relates to an amplifier, and more particularly to an audio system with power-on/off noise suppression and an audio amplifier thereof.
2. Description of Related Art
Power-on/off noises emitted by speakers on normal electronic products often annoy users. On close examination, the audio stage of the electronic products is found to be affected by a time difference resulting from power ramp down as the power is on or off, which leading to the power-on/off noises.
FIG. 1 is a block diagram illustrating a conventional audio stage circuit. In the conventional audio stage, an amplifier 120 amplifies audio signals to drive a speaker 130. The audio signals are outputted by front stage circuits 110 such as CODEC, and so on. In general, a supply voltage PVDD required by the amplifier 120 exceeds a supply voltage AVDD required by the front stage circuit 110. FIG. 2 illustrates a timing sequence relationship between a first supply voltage AVDD and a second supply voltage PVDD. Please refer to FIGS. 1 and 2 together. As the first supply voltage AVDD is increased by more than 3V, the front stage circuit 110 starts operating normally. Here, the front stage circuit 110 outputs normal audio signals. As the first supply voltage AVDD drops below 2.4V, the front stage circuit 110 is shutdown and outputs the abnormal audio signals (power-on/off noises).
When the second supply voltage PVDD exceeds 8.5V, the amplifier 120 starts amplifying the audio signals outputted by the front stage circuit 110 to drive the speaker 130. As the second supply voltage PVDD drops below 8V, the amplifier 120 is shutdown. However, since the front stage circuit 110 is shutdown before the amplifier 120 is, the power-on/off noises outputted by the front stage circuit 110 are amplified and outputted to the speaker 130. The power-on/off noises often annoy the users.